Scientific Method —

Structural materials make gains thanks to… sea shells?

Scientists have reproduced the key aspects of one of nature's top-performing …

Engineers have long taken cues from nature where, over millions of years, the highest performing designs have risen to the top. Directly mimicking nature’s structures with artificial materials, an approach called biomimetics, has given us things like gecko-inspired adhesives. Structural materials, composites in particular, have brilliant examples in nature where systems of intrinsically weak materials vastly outperform the sum of their parts thanks to how they are structured on several length scales. Nacre (abalone shell) has long been the textbook example of these composite structures, and scientists have come close to mimicking all the key aspects of the material.

Making a structural material strong is easy, but it often comes at the cost of damage tolerance and durability—carefully balancing these properties for specific applicatiosn is crucial. Nacre takes the best of both worlds, deriving its strength from an unlikely source: CaCO3, or chalk. By ordering the CaCO3 as micrometer platelettes with a small amount of a protein binder between layers, the hard and brittle CaCO3 still imparts its strength, while the hierarchical structure gives it damage tolerance. This is the same basic approach that is taken in making man-made composites, like carbon fiber reinforced plastics, although nacre is a much more efficient and effective realization of the concept.

Researchers chose Al2O3—the common ceramic alumina—and polymethymethacrylate (PMMA) in their efforts to recreate the key structures of nacre. Using a technique called freeze casting, the alumina particles are dispersed in water and a carefully controlled, directional freezing process, a layered platelette structure composed of alumina gets left behind. Removing the water, compressing the alumina, and heating it to high temperatures that are just below its melting point (called sintering) consolidates the alumina, which is then infiltrated by the PMMA to create a fully dense material.

The results were impressive—key damage tolerance properties were an order of magnitude higher than you'd expect from simply summing the contribution of the alumina and the PMMA. The structure wasn’t a dead-on replica of nacre, however, as the alumina particles were larger and the volume percentage of PMMA was higher as well, suggesting that there is still room for improvement in the process. Analysis of the fracture mechanics showed mechanics similar to nacre's, where toughening actually occurs as the crack grows. Smaller cracks ahead of the main crack front, deformation of the binder layer, and sliding of the alumina layers all consume energy, which slows the expansion of a crack.

Advanced structural materials are an often unrecognized green technology front, as weight savings from materials can be turned into higher efficiency or higher performance. Fabrication of the material presented in this research could potentially be done at a scale large enough to make something that matters out of materials that are easy to obtain. There is no doubt that an alumina/PMMA hierarchical structure is just the tip of the iceberg as far as biomimetic structural materials go.